In an interesting story on the persistence of payola, The Washington Times notes that the practice predates radio: To boost sales of sheet music, publishers used to pay bandleaders to play their songs. The story links concerns about payola to outrage over "indecency" and implicitly questions criminalization of unannounced promotional payments to radio stations:
It could be hard to build public support for a payola crackdown. In other industries, the practice is not uncommon, [George Washington University media and public affairs professor Christopher] Sterling said.
For example, food companies often pay retailers to give their products prominent placement on store shelves, he said.
"None of that is illegal. It is all an accepted way of doing business. We can argue whether it is ethical, but it's not illegal," Mr. Sterling said.